i'm pretty positive they will. according to Andrew Marlow (the guy who writes Castle), they're going to get together দ্বারা mid season. let's just hope he's not messing around with us.
I hope they get together this season. If not season 5 would also be fine.. as long as they don't pull a "bones" on us! If they do drag it along to season 5, then they'd better give us something juicy while we're waiting !